Corn-based food and snack products are very well known in the marketplace and are generally made from several types of corn (maize) and mixtures of corn with other grains. These products include corn tortillas, corn chips, taco shells, corn tostadas, corn puffs, and the like, all of which are made from nixtamalized, rehydrated, and micronized corn dough. The corn mixtures with other cereal grains may include cornsorghum, cornsoybean, and corn-amaranth, as well as other grain-enriched corn dough mixtures, including multi-grain mixtures. The term "corn dough" or "masa", as used in this application, means doughs made from corn whether or not they contain other grains or additives. Similarly, the term "corn products", as used in the instant application, means those products made from corn with or without other grains or additives.
Various methods for the preparation of a diversity of corn products are very well known in the prior art and are well documented in the literature, with some of them being proprietary techniques. The addition of flavorings, conditioners, as well as other additives and preservatives, and the choice of shape are also very well known and will depend on the characteristics desired for the product. These can serve to increase taste, nutritional values, shelf life and appearance.
As all of the above mentioned products, particularly the corn products such as corn tortillas need to be suitably cooked, various methods for carrying out such cooking have been developed in the prior art. The oldest one of said methods, which at least in Mexico may be dated back in the Tenth century, uses a so called "comal", which is a relatively flat plate made from clay, metal or ceramic material, that was heated by burning wood, charcoal or other combustible materials available at the time. In this very ancient method, the corn products, particularly corn tortillas were shaped by hand to the desired form and placed on the comal for a period of time sufficient to cook the same, turning them over at predetermined intervals with the aim of producing a uniform cooking on both sides. Obviously, the traditional methods such as the one described above, were generally batch processes.
In order to more clearly describe the type of cooking methods existent in the prior art, the cooking of corn tortillas will be taken as an example of said cooking methods. Corn tortillas, which have been a traditional food product in several Latin American countries and that are becoming more and more popular in other North American and European countries, are round flat cooked pieces of corn dough, the dimensions of which vary from one producer to the other. However, the preferred dimensions are 8 to 15 cm. in diameter, and 0.5 to 3 mm in thickness. The weight of corn tortillas is generally from 20 to 35 grams per piece. The moisture content of the uncooked corn dough used to make corn tortillas, usually ranges from 40 to 52% by weight.
During comal cooking, the temperature range of the comal varies from 180.degree. to 210.degree. C., depending on the type of comal used. When placing the tortilla for the first time in thermal contact with the comal, there is a formation of a thin vapor impervious or capping layer (a few tenths of a millimeter thick) on the side of the tortilla in contact with the comal. This layer seals the corresponding side against vapor exhaustion. Usually, the time to produce said layer will depend on the moisture content of the product and on the cooking temperature of the comal, and ranges from 20 to 40 seconds. After this time has elapsed, the tortilla is turned over to cook the other side. Cooking of the second side takes from 20 to 50 seconds. At this stage, another vapor impervious or capping layer (from 0.5 to 1.9 millimeters thick) is produced. The overall cooking of the tortilla is considered as complete once the product inflates (puffs). The degree of puffing of the tortilla during cooking, determines the efficiency of the cooking and the quality of the cooked tortilla. The capping layers are useful for retaining the correct moisture within the product and hence the overall quality needed for shelf life and handling purposes. At the end of the cooking time, moisture levels of the tortilla are reduced by from 12 to 8% by weight. The productivity of this batch type method depends on the number of tortillas able to fit on the comal at any given time, and on the thermal conductivity of the dough being cooked.
Besides the comal method described above as admitted prior art, industrial methods for continuously cooking corn tortillas have been described in several Mexican and U.S. patents. For instance, Mexican patent No. 15,798 issued during the year 1915 to Compania La India, S. A. and no longer available and therefore mentioned here as admitted prior art, for the first time introduces the continuous cooking of tortillas by providing a vertical cylindrical oven heated by wood or the lime, with a manually rotatable circular cover on which the tortillas were placed, turned over and taken off by hand.
Mexican patent No. 19,628 issued during the year 1920 to C. Celorio et al, also not available and mentioned here as admitted prior art, introduces for the first time a rudimentary automatic machine for making corn tortillas, which uses conveyor belts running within a cooking tunnel heated by means of a plurality of gas burners.
During the year 1921 a machine using a plurality of rollers heated by means of internal gas oil burners was launched to the Mexican market, without much success in view of the fact that it was extremely difficult to form the capping layers between the several pairs of heating rollers through the nip of which the tortillas had to be passed.
Mexican patent No. 45,792 issued to Fausto Celorio during the year 1947, no longer available and mentioned here as admitted prior art, introduced for the first time the use of a conveyor belt located within a cooking tunnel having a plurality of gas burners to heat the tortillas that continuously run through said tunnel on said belt. Although Celorio accomplishes a considerable improvement over the cooking methods and equipment prior to this invention, the machine still uses gas burners distributed along the path of the conveyor belt and the control of the temperature was not very efficient, whereby the percentage of discarded products used to be relatively high.
Mexican patent 104,748 issued to Fausto Celorio on Oct. 9, 1970, describes an improved cooking device which includes a series of conveyor belts having a series of burners for burning any type of fuel arranged under the stretches of the belts that transport the tortillas, and a series of perforated articulated rollers which improve the distribution of hot gases to evenly cook the tortillas. Although this machine constitutes a considerable improvement over the prior art, it still makes use of fuel burners, thus showing the same type of drawbacks shown by prior art devices, namely, difficult control of temperature throughout the device, long cooking times and occasional defective carping layers.
Mexican patent 123,272 issued to Fausto Celorio on Aug. 27, 1973, describes a cooking apparatus which includes air pre-heating means for pre-heating the air that is injected to the burners for providing oxygen for the combustion of the fuel, but without changing the principle of cooking the tortillas by means of the transportation thereof on conveyor belts to be heated by the burners.
Mexican patent 125,047 issued to Fausto Celorio on Mar. 11, 1974 describes a tortilla making machine that includes a novel take-off device for the individual tortillas but without changing the principle of cooking the tortillas on conveyor belts by means of gas burners or burners for other fuels.
Mexican patent 135,461 issued to Fausto Celorio on Mar. 16, 1977 describes an improved cooking chamber for cooking tortillas, said chamber being formed by a group of conveyor belts carrying the tortillas, the burners for gas or other fuel being arranged along the belts between the upper and lower stretches thereof, and the chamber being divided into a corresponding plurality of tunnels for forming isolated cooking sections for each conveyor belt, the partitions dividing said chamber running from the inlet end to the outlet end of said chamber and each partition being located such that each isolated cooking section includes the lower stretch of the upper belt and the upper stretch of the next lower belt. Although the capacity of production of this device is remarkably higher than that of the prior art cooking apparatus, it still preserves the same principle of cooking the tortillas with burners for fuel,
Johnson, B. R., Rooney, L. W. and Khan, M. N., Journal of Food Science, Volume 45, 1980, make reference to the cooking of corn tortillas on page 672, by stating that tortillas intended for immediate consumption and having capping layers are cooked on gas-heated griddles at a temperature of 315.degree. C., whereas "taco tortillas", that is, unpuffed tortillas, are cooked on said gas-heated griddles at a temperature of 274.degree. C., alternating sides at predetermined intervals. Again times of up to about 2 minutes have to be used and the problem of repeatedly turning over the tortillas to accomplish a uniform cooking is not eliminated.
Tonella, M. L., Sanchez, M. and Salazar, M. G., Journal of Food Science, Volume 48, 1983, also make reference to the cooking of tortillas on page 1638, but they go back to the traditional comal cooking method without suggesting any improvement over said very ancient method.
Holt, S. D., Resurreccion, R. V. R. and McWatters, K. H., Journal of Food Science, Volume 57, No. 1, 1992, describe on page 122 a method for cooking tortillas that comprises heating the tortillas on a non-stick electrically heated griddle at 190.degree. C., for 3 minutes on each side, turning over the tortillas every 90 seconds during the cooking period and pressing each time with a metal spatula to remove air pockets. This cooking method, as reported by Holt et al, is intended for the obtention of taco-type tortilla, that is unpuffed tortilla, and of course the cooking time and the need of pressing the tortilla are factors that render said method absolutely uneconomical.
Finally, U.S. Pat. No. 4,769,253 issued to Willard, M. J. on Sep. 6, 1988 and U.S. Pat. No. 4,985,269 issued to Irvin, S. A., Fedor, R. A. and Merritt, C. G. on Jan. 15, 1991, both describe cooking methods for dough pieces to produce snacks or tortilla chips, which cooking methods include the frying of the dough and not the mere heating thereof, thus considerably departing from the purpose of obtaining good quality corn tortillas or the like.
None of the above mentioned references, therefore, suggests the cooking of the corn tortillas by the use of radio frequency energy, but rather use either electrical resistive heating or the traditional heating by combustion of common fuels, either directly or through the previous heating of air which is then passed through the cooking zone.
Although some of the prior art methods described above are satisfactory for producing the capping layers on the tortilla, all of said methods show the important drawback that the conservation of moisture in the final product is deteriorated. The capping layers of the tortillas acquire the same rough surface of the hot conveyor belts due to the thermal diffusion mechanism utilized during the cooking operation. Another important drawback of these methods is the poor thermal energy transfer between the hot conveyor belt and the product, resulting in a limited productivity. The low thermal conductivity of the dough and the amount of the thermal energy wasted in heating up the conveyor belts render these processes slow and inefficient.
To overcome the drawbacks shown by the methods involving direct contact heating from a comal or from a hot conveyor belt as discussed above, and with the aim of producing good sealed capping layers, others have explored the use of microwave radiation for heating the dough. This microwave radiation methods unfortunately tend to dry the shaped dough without cooking the outside satisfactorily and produce a hard, tasteless and brittle undesirable tortilla having an unappealing appearance. In view of these discouraging results, the use of high frequency radiation has been disclosed by others not as a method of making tortillas but rather as a method for drying dough for purposes other than the production of soft edible dough products.
In view of the rather unsatisfactory results shown by the direct heating methods for the cooking of dough-based products such as corn tortillas and the unsuccessful intended use of high frequency radiation (microwaves) for said purpose, for long an effective cooking method has become indispensable, which will overcome the above described drawbacks shown by the prior art methods of cooking this type of products, particularly corn tortillas, which method has not been found prior to the advent of the present application.